The objectives of the proposed work are 1) to determine whether or not terminal deoxynucleotidyl transferase (terminal transferase) in humans is unique to thymus and lymphocytes; 2) whether 500 to 1000 foldhigher activity of terminal transferase in acute leukemic lymphoblasts (from untreated and relapse patients), compared to phytohemagglutinin stimulated normal lymphocytes, as observed previously by us, can be consistently found; 3) whether only trace or undetectable amounts of this activity are present in ALL patients during remission, in lymphocytes from normal persons and in lymphocytes from such benign diseases as infectious mononucleosis. If association of high terminal transferase activity with acute leukemic lymphoblasts can be firmly established then proposed biochemical test (which can be done on less than 50 mg cells) and immunofluorescent test (which would utilize antibodies prepared against purified terminal transferase and have the potential of detecting even a single leukemic blast cell in blood or bone marrow smears) for terminal transferase could be very useful as a diagnostic test for ALL.